We have analyzed the radar-dark parabola deposits associated with impact craters on the surface of Venus using the results of the Magellan radio experiments in both passive and active modes. The extent of mantle deposits is found to be much wider than the dark features observed in synthetic aperture radar (SAR) images. Using the emissivity data, we have classified features coinciding with the radar-dark parabolas into four types related to both differences in ejecta deposition styles for craters of different diameters, and stages in the degradation of crater-related deposits. A model for the microwave properties of the mantled surfaces is proposed. Comparison between the distribution of mantle thickness predicted by the model and the previously published independent results based on modeling of the cratering process show good agreement when the loss tangent of the mantle material is assumed to be within the 0.001--0.005 range. This value is similar to that of dry powders of terrestrial and lunar rocks. On the basis of this work we conclude that radar-dark diffuse features due to these mantles can be up to several meters thick if the loss tangent is assumed to be 0.003. Thin deposits (as little as 7 cm thick) are found to be responsible for the deposits seen in the emissivity parabolas but not observed in the SAR images. These results provide a basis for the interpretation of the character and thickness of mantle deposits, their states of degradation, processes associated with their formation and degradation, and guides to their use in establishing stratigraphic relationships and ages of underlying geologic units on Venus. The results will also be useful in interpreting the nature of surface deposits at previous landing sites and predicting the nature of deposits likely to be encountered at future landing sites.